The present invention relates to a system for converting rotational or linear mechanical movement of a body under detection to a digital signal indicative of the position, the speed, the acceleration or the like of the body.
In the past, a tachometer generator, having the same construction as a DC generator, was used for detecting the rotational speed of a rotating body. Detection of the speed of linear movement of a body was achieved by converting the linear movement into rotation and by detecting the rotational speed by means of a similar tachometer generator. Where an analog signal is used for a speed control, the output of a tachometer generator can be conveniently used as a feedback signal. Where the speed control system is composed of a digital circuitry incorporating a microcomputer or the like and hence requires a digital feedback signal, the output of the tachometer generator is converted by an analog-digital (A/D) converter into a digital signal. In such a system, the accuracy of control and the stability of the control system are dependent on the accuracy and the stability of the tachometer generator and hence advantages of the digital control are lost. Moreover, brushes in contact with a rotating member requires frequent inspection and maintenance thereof is troublesome.
In another conventional speed detection system a rotary pulse generator or a frequency generator generating an output of a frequency proportional to the rotational speed is used. The number of output pulses of the pulse generator or the frequency generator is counted and a digital signal indicative of the detected speed is produced. This system has a disadvantage in that the counting of the number of the output pulses of the pulse generator or the frequency generator has to be continued over a long time to attain a high resolution in speed detection. For instance, where a pulse generator which produces 1000 pulses per revolution is used for detecting, with a resolution of 10 bits or 0.1%, the rotational speed of 1500 r.p.m. (25 revolutions per second), the counting of the pulses takes 40 mS. As a result, response time cannot be made smaller than about 100 mS. In other words, it takes more than about 100 mS until the actual speed is made close to the reference value after the reference value is suddenly (following a step function curve) varied. Moreover, no pulse is produced by a rotary pulse generator until its rotor rotates a certain minimum angle, so that detection of a very low speed (very close to zero) is not possible. Accordingly, where the speed is to be controlled over a range extending both directions, this method of speed detection is particular unsatisfactory.